Knowing the physical properties of fluids, such as dynamic viscosity, local (dynamic) surface tension and surface dilational elasticity is essential when designing fluid processing equipment. Yet these properties vary to a considerable degree depending on the precise operating conditions which the fluid encounters. In particular, the design and operation of setting and drying equipment of coating processing units depend on a knowledge of the physical properties of the process fluids at operating conditions. It is frequently difficult to obtain such data, which varies throughout the setting or drying process. Thus, physical properties, such as viscosity and surface tension are extrapolated from conditions that are very different, and surface dilational elasticity is typically guessed at. Such approaches, however, interject error into the design and operating calculations and create deficiencies in the coating setting and drying processing units. Furthermore, the ability to measure such properties as viscosity of coatings on-line during a coating operation contributes to determining the quality of the coating as it is being manufactured. The present invention, under certain circumstances, can be used to help process quality.
U.S. Pat. Nos. 4,674,322, 3,550,433, 4,953,389, 4,884,437; 5,303,030 and 5,317,387 are illustrative of the prior art relating to the present invention. In U.S. Pat. No. 4,674,322 an instrument for simultaneously measuring viscosity, surface tension and density of a liquid mixed with a gas is disclosed, but it operates on the principle of an harmonic oscillator and cannot be used for thin liquid films. None of the above patents, however, are directed to a method of determining the properties of viscosity and surface tension and surface dilational elasticity of a coating on a support, or perform measurements on thin film. On the other hand, U.S. Pat. No. 3,550,433 describes an apparatus to measure viscosity of drying films by immersing a wire in the liquid film and measuring the tension on the wire as it moves through the film. The principle of the present invention applies electrostatic (and not mechanical) forces and optical or other non-contact (not mechanical) means to measure the liquid film properties.
Heckl et al. "Electric-field-induced domain movement in phospholipid monolayers", in Thin Solid Films, V. 159, pp. 125-132 (1988) describe: an apparatus consisting of a rod electrode positioned perpendicularly to a plate on which a liquid film rests; and a method of measuring, by means of fluorescence microscopy, the radius of the growing circular wave when the electric field is turned on, and determining the viscosity of the film. However, in the apparatus and method described, it appears to be essential that lipid monolayers be present at the free surface, that are attracted or repelled by the electrostatic field, and the equation used to calculate this viscosity depends on such parameters as the size of gel phase domains. In contrast to Heckl et al. the apparatus and method of the present invention are applied to films with properties that are uniform in the plane of the support and the equation used to measure viscosity is completely different and based on different assumptions.
Another aspect of this invention is that, if the viscosity of the coated film is already known (by a means other than this invention), then it is possible to use the invention to measure the thickness of the wet film. U.S. Pat. Nos. 4,169,319 and 4,776,099 offer mechanical means of making this measurement by using, respectively, disks with calibrated notches of different depths and a circular disk that rotates eccentrically. Wet thickness is determined by the location (a notch and angle on the disk, respectively) at which the coating stops wetting the disk. U.S. Pat. No. 3,869,984 teaches a process of controlling wet film thickness that includes predicting film thickness by combining the results of measurements of the splitting and sliding shear forces exerted on a device facing a coating roll, over which the coated support is wrapped and moving, and in which the device wets the coating. Lyu and Mudawar ("Simultaneous measurements of thickness and temperature profile in a wavy liquid film falling freely on a heating wall", Exp. Ht. Transf., V. 4, pp. 217-233, 1991) published an article demonstrating a device that measures the thickness of a wavy film. The invention consists of a wire that penetrates the liquid film and is made of a material that changes resistance with temperature; the instantaneous resistance of the wire is made, from which they are able to predict thickness and temperature profile instantaneously. All of these inventions require contact and penetration of a device into the wet film. There are at least two methods that do not penetrate the film which are used for some dry coating, and probably can be used with wet coatings (see Cross, "Thickness measurements--what do they mean?", Pltng & Surf. Fnshng, November 1979, pp. 22-28). These measure, respectively, eddy currents and magnetic attraction to calculate film thickness. The method claimed herein is non-penetrating and uses an electric field instead and measures the resulting displacement of the free surface from which wet film thickness can be calculated.
A problem with many of the above references is that they do not teach the measurement of viscosity of different layers in a coating traveling on a moving web.
Dynamic viscosity of a liquid and dynamic surface tension of a free surface or interface between a liquid and a gas are concepts that are familiar to most knowledgeable practitioners in the area of fluid mechanics and will not be discussed here. (For brevity, the modifier "dynamic" will be dropped when referring to both properties.) On the other hand, the surface dilational elasticity is not as well known and will be discussed here. (A good textbook in this field is the one edited by E. H. Lucassen-Reynders: "Anionic Surfactants: Physical Chemistry of Surfactant Action", Surfactant Science Series, Vol. 11, Marcel Dekker, Inc., 1981.) Surface dilational elasticity is a measure of the free surface's opposition to being deformed in its own plane (by stretching and compressing). Pure liquids such as pure water do not oppose stretching or compressing so their surface dilational elasticity is zero; however, small amounts of contaminants in the liquid such as dirt or surfactants (including soap) tend to migrate to the free surface and build up the necessary stresses to oppose the motion in some degree. In industrial processes, an interface that possesses this property is frequently useful because waves that would otherwise develop at such a free surface can be damped or even eliminated, providing greater control or uniformity. For liquid films that are coated, such as those found in the painting and coating industry, the free surface's ability to move (or oppose motion) can have a large influence on the flow of the entire liquid film as the ratio of the free surface to liquid volume is so high. Thus the finished quality of the painted or coated film can depend greatly on this property. The importance of the stresses induced at free surfaces is well recognized in the industry. (See, for instance, "Static and dynamic surface tension of aqueous mixtures of surfactants and colloidal lattices", a presentation by I-M. Tricot at the 7th International Coating Process Science and Technology Symposium held in Atlanta, Ga. on Apr. 17-21, 1994.)
Surface dilational elasticity is not an intrinsic property of material in the way that viscosity and static surface tension are, but a manifestation of how various physical processes combine to resist or oppose flow at the free surface. It is defined as the rate of change of the surface tension with respect to the rate of change of the natural logarithm of the surfactant's surface concentration. The principal properties involved are the build-up of surface tension gradients when the surfactant concentration at the free surface varies, and the ability of the surfactant to reach the free surface (by diffusion or convection) from the bulk of the liquid and the rate of distortion of the free surface. However, as the interaction between these properties is very complicated, it is usually more convenient to measure surface dilational elasticity directly.
There are a number of methods of measuring surface dilational elasticity in pools of liquid or for liquids flowing down inclined planes. These methods depend on exciting waves in the film and measuring the degree to which they damp. Some of these methods are described in the aforementioned book by Lucassen-Reynders. They typically rely on producing waves through an oscillatory excitation, including electrostatic excitations by a method called electrocapillarity. (See C. H. Sohl, K. Miyano and J. B. Ketterson, "Novel technique for dynamic surface tension and viscosity measurements at liquid-gas interfaces", Rev. Sci. Instrum., V. 49, p. 1669, 1978.) Although the excitation is similar to the one of the present invention, this other method excites a stationary pool of liquid with a sinusoidal excitation in time. Electrocapillarity has also been used before to excite waves in liquids flowing down an inclined plane (see S. J. Weinstein, J. M. Baumlin and J. Servant, "The propagation of surface waves in flow down an oscillating inclined plane", AIChE Journal, V. 39, pp. 1113-1123, 1993), but the purpose there was simply to measure growth of waves of uncontaminated liquids, and not to measure physical properties, and the excitation was oscillatory. In contrast, the method used here applies a steady electrostatic field that is spatially non-uniform to a liquid film, and relies on the dynamics of that film (which are different from those of a liquid pool or of flow down an inclined plane) to determine the viscosity, surface tension and surface dilational elasticity of the liquid film, which is either stationary or moving at a constant speed under the electrodes used to impose the field.
Steady state spatially non-uniform electrostatic fields create non-uniform normal stresses at the free surface which tend to pull the liquid. Because the pulling action is not uniform, the liquid tends to flow in the direction where the field is strongest and accumulates in this region. This approach has been used to change the thickness of liquid films flowing down a wall, but it is usually applied to enhance condensation of the liquid. For instance, Joos and Snaddon ("Electrostatically enhanced film condensation", J. of Fluid Mechanics, V. 156, pp 23-38, 1985) created such an electric field in a condenser tube by inserting asymmetrically a metal rod into the tube and applied high voltage to the rod. However, this type of application is very different as it involves liquid flowing down the inside of a tube, liquid that is not contaminated and so it has no surface dilational elasticity, and its focus is on increasing heat transfer rates, not on measuring material properties.
When a film is placed horizontally on a support under a spatially non-uniform electric field, other forces appear, such as gravity and surface tension, which tend to oppose this accumulation of liquid. Forces due to viscosity resist the motion, and eventual surface tension gradients (equal to the product of the surface dilational elasticity and the surfactant surface concentration gradient) build up sufficiently to oppose the flow. It is by trying to find relatively simple ways in which these forces interact that is applied in this invention to measure viscosity, surface tension and surface dilational elasticity.